1. Field of the Invention
The invention concerns an ultrasonic depth sounder or sensor with sectorial electronic scanning utilizing the sound emission technique generally known under the name of "phased array", where a single group of transducer elements is excited at each emitting-receiving sequence with predetermined delays between the transducer elements of the group. The invention also concerns an echograph incorporating a novel type of sounder or sensor.
2. Description of the Prior Art
A known echograph with electronic sectorial scanning comprises a sounder or sensor incorporating an array of distinct transducer elements aligned according to a relatively short straight segment. By way of example, the dimensions of the bar or strip of piezo electric transducer elements are currently 20 mm long and about 15 mm wide. The number of transducer elements is generally smaller than 100 and more often comprises between 30 and 70. As compared with strip systems comprising a higher number of elements and among which a different group of transducer elements is selected at each emitting-receiving sequence, all the transducer elements of the group of this known echograph are called at each emitting-receiving sequence. The transducer elements are excited with predetermined delays between them, and then the total of the echos perceived by these same elements is calculated, preferably by applying the same lag or delay law to these echos. Upon transmitting, the delay law conditions both the orientation of the ultrasonic beam and the focussing characteristics of this beam. Upon receiving, the same lag or delay law applied between the different signal processing channels prior to their addition, creates a dynamic focalization and privileges the reception according to a certain direction. It can thus be conceived that the electronic part of this type of echograph is very complex and expensive. Furthermore, it is very difficult to achieve a correct scanning on a wide angular sector. In fact, the greater the slope angle with respect to the piezo electric strip materializing the transducer element grouping, the greater must be the delays between the elements. By way of example, a beam deflected by 45.degree. with respect to the emitting plane of the strip requires delays which must be able to reach ten microseconds. The value of these delays must be controlled with great accuracy, of about 20 nanoseconds. Realizing these delays raises complicated technical problems. Furthermore, it has been observed that the sensitivity of a piezo electric element rapidly decreases upon reception, when the angle of incidence of the corresponding beam increases.